[PATCH v3 2/6] ubsan: Split "bounds" checker from other options

Kees Cook
2020-01-16T01:24:28Z

In order to do kernel builds with the bounds checker individually
available, introduce CONFIG_UBSAN_BOUNDS, with the remaining options
under CONFIG_UBSAN_MISC.

For example, using this, we can start to expand the coverage syzkaller is
providing. Right now, all of UBSan is disabled for syzbot builds because
taken as a whole, it is too noisy. This will let us focus on one feature
at a time.

For the bounds checker specifically, this provides a mechanism to
eliminate an entire class of array overflows with close to zero
performance overhead (I cannot measure a difference). In my (mostly)
defconfig, enabling bounds checking adds ~4200 checks to the kernel.
Performance changes are in the noise, likely due to the branch predictors
optimizing for the non-fail path.

Some notes on the bounds checker:

- it does not instrument {mem,str}*()-family functions, it only
instruments direct indexed accesses (e.g. "foo[i]"). Dealing with
the {mem,str}*()-family functions is a work-in-progress around
CONFIG_FORTIFY_SOURCE[1].

- it ignores flexible array members, including the very old single
byte (e.g. "int foo[1];") declarations. (Note that GCC's
implementation appears to ignore _all_ trailing arrays, but Clang only
ignores empty, 0, and 1 byte arrays[2].)

[1] https://github.com/KSPP/linux/issues/6
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92589

Suggested-by: Elena Petrova <lenaptr@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
---
lib/Kconfig.ubsan | 29 ++++++++++++++++++++++++-----
scripts/Makefile.ubsan | 7 ++++++-
2 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index 9deb655838b0..48469c95d78e 100644
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -2,7 +2,7 @@
config ARCH_HAS_UBSAN_SANITIZE_ALL
bool

-config UBSAN
+menuconfig UBSAN
bool "Undefined behaviour sanity checker"
help
This option enables the Undefined Behaviour sanity checker.
@@ -10,9 +10,10 @@ config UBSAN
behaviours at runtime. For more details, see:
Documentation/dev-tools/ubsan.rst

+if UBSAN
+
config UBSAN_TRAP
bool "On Sanitizer warnings, abort the running kernel code"
- depends on UBSAN
depends on $(cc-option, -fsanitize-undefined-trap-on-error)
help
Building kernels with Sanitizer features enabled tends to grow
@@ -25,9 +26,26 @@ config UBSAN_TRAP
the system. For some system builders this is an acceptable
trade-off.

+config UBSAN_BOUNDS
+ bool "Perform array index bounds checking"
+ default UBSAN
+ help
+ This option enables detection of directly indexed out of bounds
+ array accesses, where the array size is known at compile time.
+ Note that this does not protect array overflows via bad calls
+ to the {str,mem}*cpy() family of functions (that is addressed
+ by CONFIG_FORTIFY_SOURCE).
+
+config UBSAN_MISC
+ bool "Enable all other Undefined Behavior sanity checks"
+ default UBSAN
+ help
+ This option enables all sanity checks that don't have their
+ own Kconfig options. Disable this if you only want to have
+ individually selected checks.
+
config UBSAN_SANITIZE_ALL
bool "Enable instrumentation for the entire kernel"
- depends on UBSAN
depends on ARCH_HAS_UBSAN_SANITIZE_ALL

# We build with -Wno-maybe-uninitilzed, but we still want to
@@ -44,7 +62,6 @@ config UBSAN_SANITIZE_ALL

config UBSAN_NO_ALIGNMENT
bool "Disable checking of pointers alignment"
- depends on UBSAN
default y if HAVE_EFFICIENT_UNALIGNED_ACCESS
help
This option disables the check of unaligned memory accesses.
@@ -57,7 +74,9 @@ config UBSAN_ALIGNMENT

config TEST_UBSAN
tristate "Module for testing for undefined behavior detection"
- depends on m && UBSAN
+ depends on m
help
This is a test module for UBSAN.
It triggers various undefined behavior, and detect it.
+
+endif # if UBSAN
diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
index 668a91510bfe..5b15bc425ec9 100644
--- a/scripts/Makefile.ubsan
+++ b/scripts/Makefile.ubsan
@@ -5,14 +5,19 @@ ifdef CONFIG_UBSAN_ALIGNMENT
CFLAGS_UBSAN += $(call cc-option, -fsanitize=alignment)
endif

+ifdef CONFIG_UBSAN_BOUNDS
+ CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
+endif
+
+ifdef CONFIG_UBSAN_MISC
CFLAGS_UBSAN += $(call cc-option, -fsanitize=shift)
CFLAGS_UBSAN += $(call cc-option, -fsanitize=integer-divide-by-zero)
CFLAGS_UBSAN += $(call cc-option, -fsanitize=unreachable)
CFLAGS_UBSAN += $(call cc-option, -fsanitize=signed-integer-overflow)
- CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
CFLAGS_UBSAN += $(call cc-option, -fsanitize=object-size)
CFLAGS_UBSAN += $(call cc-option, -fsanitize=bool)
CFLAGS_UBSAN += $(call cc-option, -fsanitize=enum)
+endif

ifdef CONFIG_UBSAN_TRAP
CFLAGS_UBSAN += $(call cc-option, -fsanitize-undefined-trap-on-error)
--
2.20.1